Preparation and Characterization of Reduced Graphene Oxide / Titanium Dioxide Composites by Hydrothermal Method

In the present work, reduced graphene oxide/Titanium dioxide (rGO/TiO2) composites (1:1 and 1:2 wt %) have been synthesized by the hydrothermal method using graphene oxide (GO) and commercial TiO2 as precursors. Previously, we prepared the GO, in the way optimizing and making safer, the Hummers route. We have chosen the hydrothermal method to prepare the composites because it offers several advantages: 1) It consist of a very simple experimental setup, 2) it utilizes only water, instead of Hydrazine or Sulfonate used as chemical reductants in traditional methods, avoiding the incorporation of un-willing impurities into GO sheets, 3) the temperature and pressure condition reached in the closed hydrothermal system have promoted the recovery of π-conjugation after dehydration diminishing defects concentration and increasing the degree of reduction of the GO sheets, and 4) this system is compatible with industrial batch production. The structure, surface morphology, chemical composition and optical properties of GO, TiO2 and rGO/TiO2 composites have been analyzed using, TEM, FTIR, Raman- and XPS-spectroscopy. TEM micrographs show that the TiO2 nanoparticles are non-homogenously adsorbed onto the GO sheets. FTIR spectra of the rGO/TiO2 composites suggest that during the hydrothermal process the GO sheets get reduced. Raman spectra suggest that TiO2 remains with anatase structure even after the hydrothermal process. The C 1s XPS spectra of the rGO/TiO2 composites have shown a significant decrease of oxygenated carbon related signals, confirming that most of the oxygenated groups were successfully removed. Based on these characterization results we infer that, GO sheets of good quality have been successfully synthesized and the GO sheets have been partially reduced via the TiO2 nanoparticles anchored during the hydrothermal process.


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Abstract-In the present work, reduced graphene oxide/Titanium dioxide (rGO/TiO2) composites (1:1 and 1:2 "in weight" (in wt) have been synthesized by the hydrothermal method using graphene oxide (GO) and commercial TiO2 as precursors. Previously, we prepared the GO, in the way optimizing and making safer, the Hummers route. We have chosen the hydrothermal method to prepare the composites because it offers several advantages: 1) It consist of a very simple experimental setup, 2) it utilizes only water, instead of Hydrazine or Sulfonate used as chemical reductants in traditional methods, avoiding the incorporation of un-willing impurities into GO sheets, 3) the temperature and pressure condition reached in the closed hydrothermal system have promoted the recovery of π-conjugation after dehydration diminishing defects concentration and increasing the degree of reduction of the GO sheets, and 4) this system is compatible with industrial batch production. The structure, surface morphology, chemical composition and optical properties of GO, TiO2 and rGO/TiO2 composites have been analyzed using, TEM, FTIR, Raman-and XPS-spectroscopy. TEM micrographs show that the TiO2 nanoparticles are nonhomogenously adsorbed onto the GO sheets. FTIR spectra of the rGO/TiO2 composites suggest that during the hydrothermal process the GO sheets get reduced. Raman spectra suggest that TiO2 remains with anatase structure even after the hydrothermal process. The C 1s XPS spectra of the rGO/TiO2 composites have shown a significant decrease of oxygenated carbon related signals, confirming that most of the oxygenated groups were successfully removed. Based on these characterization results we infer that, GO sheets of good quality have been successfully synthesized and the GO sheets have been partially reduced via the TiO2 nanoparticles anchored during the hydrothermal process.

I. INTRODUCTION
The research and development of new materials with better photocatalytic properties have drawn increasing attention driven by the motivations of exploring new materials with better photocatalytic properties. The TiO2 is a semiconductor with a direct energy bandgap (3.2 eV) in the UV range of the electromagnetic spectrum [1]. The TiO2 is the material most commonly used as a photocatalyst due to chemical and thermal stability, high efficiency and low cost, etc [2][3][4][5][6]. The TiO2 exists in nature in two tetragonal forms, rutile and anatasa structures, and in a rhombic form called brookita. However, the anatasa structures seems to be more active photocatalytically probably due to differences in the extent and nature of the surface hydroxyl groups [2]. This semiconductor TiO2 is synthesized by the hydrolysis of the TiCl4 in a hot flame. This technique produces TiO2 of about 4:1 anatase to rutile crystal structures ratio with specific surface area (~50 m 2 /g) [3,4].
The photochemical application of TiO2 photocatalysis is invariably affected by the surface properties of the TiO2 particle and the photoinduced effect is affected by quantum size [5]. In photocatalisys application, commercial TiO2 Degussa P25, is commonly used for example in: hydrogen production [6,7], water purification [8,9], and air detoxification [10,11].
In the past few decades, in order to increase the efficiency of the redox reaction involved in the photocatalysis process, the TiO2 particles has been anchoring onto large-surfacearea materials such as mesoporous, zeolites and low dimensional carbon-based materials [5,[12][13][14].
In this context graphene, one-atom-thick planar sheet of carbon atoms densely packed in a honeycomb crystal lattice, has emerged as a high potential material due to its high surface area (theoretical value 2630 m 2 /g) [15], mechanical flexibility, optical transparency, electrical conductivity [16] and good interface contact with adsorbents among other properties. The structure and properties of graphene depend on the synthesis method. Up today, several techniques have been established for graphene synthesis but mechanical cleaving (exfoliation) [17], chemical exfoliation [18], chemical synthesis [19], and thermal chemical vapor deposition (CVD) [20] are the most commonly used methods.
Nowadays, the only route that affords graphene-based sheets in considerable quantities relies on the chemical conversion of graphite to graphite oxide. Graphite oxide often called graphitic acid or graphitic oxide is a polycyclic aromatic hydrocarbon oxide interrupted by epoxides, alcohols, ketone carbonyls, and carboxylic groups [20][21][22][23].
Brodie et al. [24] first demonstrated the synthesis of graphite oxide in 1859 by adding a portion of potassium chlorate (KClO3) to slurry of graphite in the presence of fuming nitric acid. It was until 1898 that Staudenmaier improved Brodie's method by replacing about two thirds of Preparation and Characterization of Reduced Graphene Oxide/Titanium Dioxide Composites by Hydrothermal Method fuming HNO3 by concentrated H2SO4 increasing the acidity in the mixture besides he added KClO3 as the oxidant agent in nitric acid, minimizing in this way the risk of explosion [25]. However, in 1958 Hummers and Offeman [26] developed a method for the synthesis of graphite oxide based in all former developments but with key improvements. It consists of carrying out a reaction of graphite powder with KMnO4 and NaNO3 as oxidant agents in concentrated H2SO4. Since then, this method has been the most important and widely used for the synthesis of graphite oxide. Additionally, the graphite oxide can be exfoliated in many polar solvents and dispersed particularly well in water obtaining graphene oxide (GO) nanosheets.1q` 1 Since the graphene based composites can be processed from aqueous and polar solvents, they offer the convenience to cast thin films for various technological advances. For example, the anchoring of catalyst particles on carbon nanostructures can provide new ways to increase the surface area and improve: the catalysis performance of energy conversion devices, the enhanced electrocatalytic activity of semiconductor particles dispersed on carbon sheets of fuel cells [27,28], TiO2/carbon nanotube composites have been established as viable potential photocatalysts for use in both water and air purifications [9,29]. The synergetic effect of carbon nanotubes on photocatalyst enhancement, in which carbon nanotubes act as the electron sink for the hindrance of charge carrier recombination [9] or as the photosensitizer to generate a greater concentration of electron/hole pairs [29] has been previously demonstrated. Carbon nanotubes (CNT) also behave as impurities, resulting in the formation of Ti-O-C bonds and, therefore, expanding the light absorption to longer wavelengths [30]. For this reason, the two dimensional carbon nanostructures such as graphene can also potentially serve as a support material on which semiconductor particles can be anchored to improve the performance of optoelectronic and energy conversion devices.
The aim of this research work is to incorporate oxygen containing groups on carbonaceous sheets via oxidation with strong acids to form a stable GO suspension in water and anchor TiO2 particles on the GO sheets by means of a hydrothermal method to form rGO/TiO2 composites with different TiO2 concentrations. Microscopy and spectroscopy techniques are used to analyze the oxidation-reduction degree of the composites.

II. EXPERIMENTAL PART
A. Synthesis of GO4 GO was synthesized by carrying out the oxidation process of graphite powder in accord to the "safer modified Hummers' method" [31]. We started the synthesis of graphite oxide by using graphite powder (Bay carbon, spectroscope powders, Bay City, Michigan 48706, ~100 μm) and added a mixture of graphite and KMnO4 powders (3:18 g; 3:6 in wt) into an H2SO4/H3PO4 solution (360:40 ml; 9:1 v/v proportion). One of the sub-products of the KMnO4 and H2SO4 reaction is Mn2O7 which detonates at temperatures above 55 °C [32] thus, to prevent an explosion we cooled down both mixtures before mixing them. Consequently, only a slightly exothermal reaction took place at 35-40 °C and the solution turned purple-brown color. Next, the mixture was heated at 50 °C with continuous stirring for 12 h, at this point, the mixture turned dark brown indicating that the graphite oxidation reaction already took place. During this thermal process the mixture undergoes remarkable changes: gases are given off from the interior of the substance, which swells up the system in a most singular manner separating the carbonaceous sheets and reducing them to the minutest state of division which have the appearance and the structure of lamellar graphite [24], larger surface areas of the carbonaceous sheets are in contact with the KMnO4 increasing their oxidation state. Based on these experimental results we infer that 12 h of the oxidation process is the appropriate time in which an efficient graphite oxidation reaction takes place.
Continuing with the rinsing procedure, the mixture was let cool down to room temperature, afterwards 400 ml of DI water in ice form was added to decrease its viscosity and avoid overheating [33,34]. Later on, 9 ml of H2O2 (H2O2/DI water, 30% v/v) was added to the mixture to make easier the removal of metal salts such as permanganate and manganese residuals from the mixture, now this solution turned to a bright yellow color. The solution was centrifuged at 1500 rpm for one hour and the supernatant decanted away. The remainder was repeatedly rinsed in continuous succession with: 1) 200 ml of DI water, 2) 200 ml of HCl at 30% v/v and 3) 200 ml of ethanol to remove the byproducts (e.g., potassium-containing compounds) that can cause an acute explosion [35]; after each rinsing step, the solution was centrifuged at 4000 rpm for 2 h and the supernatant decanted away. Then, the remainder obtained after the rinsing procedure was coagulated with 40 ml of ether. Later on, the resulting suspension was filtered through a PTFE membrane with 0.45 μm pore size. Finally, the solid material obtained on the filter was vacuum-dried for overnight at room temperature. To remove water from graphite oxide, we carried out a thermal treatment at 60 °C (temperature below the one of graphite oxide reduction) under nitrogen flux for 2 h. The final product in powder form weighted 6.8 g. By considering a graphite initial weight of 3 g, it is estimated that about 80 % of carbon is linked to oxygen in different ways, including -OH groups.
Since it is already known that chemically both graphite oxide and GO have similar or identical structures; both possess stacked structures with chemical functionality on their basal planes and at their edges [32]. The only difference between them is the number of stacked layers; GO possesses a monolayer or just a few stacked layers, while graphite oxide contains a greater number of stacked layers [32]. Taking into account, that the formation of oxygenated functional groups in graphite oxide makes them easier to exfoliate it into monolayers of GO, we dispersed 1 mg of graphite oxide powder in 100 ml DI water by sonication for 10 min to obtain GO nanosheets. Then, the prepared samples were analyzed by microscopic and spectroscopic techniques.

B. Synthesis of rGO/TiO2 Composites
The rGO/TiO2 composites were prepared by means of a hydrothermal treatment. First, 0.05 g of graphite oxide was added to 50 ml of DI water and the mixture was sonicated for 2 h to achieve a uniform dispersion of GO. Previously, commercial TiO2 nanoparticles (P25 TiO2, Degussa) were thermally treated at 550 °C for 2 h to obtain clean TiO2 particles surface. Then, 0.05 g of TiO2 was added slowly to the GO dispersion solution and further stirred for 1 h to ensure complete mixing. This mixture was transferred to a Teflon-lined autoclave and heated at 150 °C under static condition for 5 h then, the solution was let cool down to room temperature. Later on, it was centrifuged at 18000 rpm for 1 h and the supernatant was decanted away. This product was rinsed with DI water several times, centrifuged at 18000 rpm and dried in an oven at 60 °C for 6 h. The final product is the rGO/TiO2 composite at 1:1 in wt. By the same procedure, but doubling the amount of TiO2, it was obtained the rGO/TiO2 composite 1:2 in wt.

A. Characterization Techniques
The surface morphology of the GO, TiO2 and rGO/TiO2 samples was analyzed using a high-resolution transmission electron microscope (HRTEM) JEOL JEM-2100F, operated at 200 kV acceleration voltage. FTIR spectra (400-2000 cm -1 ) were collected with a Spectrum One Perkin Elmer system with an ATR accessory using a scan velocity of 8 nm/s. Raman spectra were recorded with a Scientific DXR, Thermo SCIENTIFIC Raman system with a He-Ne laser (λ = 632.8 nm) with 10 mW of power as excitation source. The entrance and exit slits aperture was 50 µm. An optical microscope is coupled to this system and 10X magnification was used to select the region of interest. An X-ray photoelectron spectroscopy (XPS) system K-Alpha+ Thermo Scientific, with a Constant Analyzer Energy (CAE) was used to give insights about the chemical nature of the surface of the composites. XPS gives quantitative information about chemical groups and the chemical shift in XPS is a measure of the valence charge on the atom of interest. XPS spectra were obtained by irradiating the samples with an Al Kα (1486.6 eV) X-ray beam in an analyzing chamber with pressure of 2x10 -9 Torr.

B. Morphological characterization
The surface morphology of the synthesized GO, TiO2 and rGO/TiO2 composites was analyzed by means of TEM (see Figure 1(a)). It can be observed that the morphology of GO is sheet like. The sheets look translucent, well defined with some wrinkles on the surface and with no folding on the edges. In these micrographs it can be observed some imperfections, dark and light zones and some dark points on the surface that cause some irregularities on the plane surface. These imperfections are related with the existence of carbon atoms with sp 3 hybridization that constitute the hydroxyl or epoxy groups on the GO sheets [36]. In the inset of this figure it is shown an HRTEM image of the GO. Figure 1(b) shows TEM micrographs of TiO2 (Degussa P25). It is observed that, as expected, the morphology of TiO2 powder consists of agglomerations of particles of different shapes and sizes. In the inset of Figure 1(b), it is seen a cluster of particles with irregular shape and length of about 20 nm. Figure 1(c) shows TEM micrographs of the rGO/TiO2 composite in proportion 1:1 in wt. It is observed that TiO2 particles cover the whole GO surface. In Figure  1(d) a TEM micrograph of the rGO/TiO2 composite 1:2 in wt is shown. Once again a conglomeration of TiO2 particles is observed, this conglomeration causes a nonhomogeneous distribution of TiO2 on the GO sheets. According to the insets in Figure 1(c,d), the size of the TiO2 particles in both composites is about 40 nm due to the same conditions of temperature and pressure used in the hydrothermal process. Additional information about the elements concentrations can be observed in the EDS mapping images: Figure 2  Finally, Figure 3 shows the elemental mapping images of the rGO/TiO2 composite 1:2 in wt. In (a) the gray image of the region under analysis, in (b) Carbon in blue, (c) Oxygen in yellow, (d) Titanium in pink and (e) the color overlay of the three mappings. Thus, we can infer that the Ti atoms are non-homogeneously adsorbed on the GO sheets.  Figure 4(a) shows an FTIR transmission spectrum recorded from GO in the range from 4000 to 700 cm -1 . In this spectrum appear different peaks that are assigned to vibrational modes of oxygen-containing functional groups; for a better visualization see points (A), (B) and (C) on the inset of this figure. Additionally, C=C and C-O vibrations of GO are also present, as expected. It is worthy to mention that the FTIR spectrum of GO recorded after one year is very similar to the one recorded from the fresh GO, this means that GO is stable with respect to aging.

D. FTIR spectroscopy
In Table 1 the measured peaks positions, the reported ones, and their assignments according to the literature are listed. There is a good concordance between the two groups. We infer then, that the GO nanosheets are covered by functional groups such as hydroxyl (-OH), epoxy (C-O-C) and carboxylic (-COOH) groups. It is worthwhile to mention that the FTIR spectrum of GO recorded after one year is very similar to the one recorded from fresh GO, this means that GO is stable with respect to aging.  [32,43] In Figure 4(b) the FTIR transmission spectrum of TiO2 is shown; the 3 peaks located at 3440 cm -1 , 1630 and 1430 cm -1 are associated to a tension vibrational mode of H-OH, to the flexion vibrational mode of water and to vibrational modes of residual C-H organic groups, respectively. These residual groups come from the reactants used during the synthesis process and to the TiO2 nature [44]. On the other hand, in the range 800 -400 cm -1 the bands of TiO2 with anatasa structure are present: the one at 466 cm -1 associated with the Ti-O vibrational mode, and the bands close to 515 cm -1 and 715 cm -1 , assigned to vibrational modes of the Ti-O-Ti bonds [45].
In Figure 4(c) transmission FTIR spectra of rGO/TiO2 1:1 in wt and rGO/TiO2 1:2 in wt are shown. As mentioned before, these spectra suggest that during the solvothermal process the GO gets reduced, that is, the GO loses oxygenated groups and consequently the sp2 Carbon bonds band increases in intensity becoming reduce graphene oxide (rGO) sheets. This effect is observed clearly in Figure 4(d) by the intensity increase of the band at 1600 cm -1 , corresponding to C=C bonds. Besides, TiO2 binds to the GO surface by means of C-O bonds clearly seen by the presence of the band at 1734 cm -1 of the composites spectra.

E. UV-Vis spectroscopy
We used the UV-Vis spectroscopy technique to monitor the reduction of GO. In Figure 5, normalized UV-Vis absorbance spectra (in the range: 200-800 nm) of GO, rGO/TiO2 1:1 in wt and rGO/TiO2 1:2 in wt composites are shown. In the GO spectrum it is observed an absorption band with maximum at 230 nm characteristic of GO due to transitions π-π* in its graphitic structure (interactions of orbitals π in bonding C=C and π* in bonding C-C) [46]. Also it is observed, on the adsorption edge a shoulder at 300 nm corresponding to oxygenated groups. On the other hand, the absorbance spectrum of the rGO/TiO2 composite 1:1 in wt presents an absorption band with maximum at 264 nm, that is 34 nm red shifted with respect to 230 nm of the GO peak and in the spectrum of rGO/TiO2 1:2 in wt the corresponding maximum appears at 254 nm, that is 24 nm red shifted with respect to the one of GO peak (see the inset in Figure 5, that shows the top parts of the bands). These red shifts are due to reduction of GO [47], suggesting that the GO in both composites might be reduced and the aromatic structure might be restored gradually. The reduction of the GO when forming the rGO/TiO2 composites 1:1 and 1:2 in wt is probably due to superheated H2O that promotes an acid-catalyzed reaction of organic compounds because it generates a sufficiently high H + concentration to remove oxygenated groups on the surface GO sheets. In other words, the GO reduction is indicative of the restoration of the π-conjugation network within the graphene nanosheets. These results obviously demonstrate the significant influence of graphene oxide on the optical characteristics in which increasing graphene oxide amount narrows the band gap of TiO2. Similar to the case of TiO2-CNT, C-doped TiO2 or TiO2-chemically converted graphene composites, the phenomena in this study could be ascribed to the formation of Ti-O-C chemical bonding in the prepared composites [48][49][50][51].

F. Raman spectroscopy
Raman spectroscopy is a valuable tool to characterize carbon based materials. Figure 6(a) shows the Raman spectrum of graphite oxide. It consists of a first order (1100-1800 cm -1 ) and second order (2500-3100 cm -1 ) regions [52,53]. In the former region appears the D band at 1350 cm -1 assigned to the presence of defects such as: bond-angle disorder, bond length disorder, vacancies, and edge defects and a G band at 1580 cm -1 that corresponds to the E2G vibration mode of a crystal with 6ℎ 4 symmetry [36]. Also, in the second order scattering range appears at 2700 cm -1 the 2D band also called D+G band or G' band which is an overtone of the D band [36]. The Raman spectrum shown in Figure 6(a) is very similar to the one reported in the literature by Kuila, et al. [41] therefore, we infer that the processes we have performed to graphite powder in order to obtain GO are appropriate. Furthermore, Figure 6(b) shows overlapped Raman spectra in the range 100-900 cm -1 corresponding to: as received TiO2, TiO2 thermally treated at 550 ˚C and to TiO2 solvothermally treated at 550 ˚C. The Raman scattering bands appearing at 151, 194, 395, 516 and 640 cm -1 correspond to TiO2 with anatase phase and the low intensity band at 445 cm -1 corresponds to the rutile phase [54] which means that a low contribution of the rutile phase is present in the TiO2 anatase phase. As can be seen the three spectra are very similar except that the one of TiO2 solvothermally treated is intensified. Figure 7 shows overlapped Raman spectra (100-3500 cm -1 ) of the rGO/TiO2 composites: 1:1 and 1:2 in wt. In the range 100-650 cm -1 appear the Raman lines at 144, 396, 509 and 632 cm -1 . The peaks positions are slightly shifted with respect to the ones of TiO2 as received. In the range 1100-1800 cm -1 appear the Raman lines corresponding to GO: the D band at 1347 cm -1 and the G band at 1593 cm -1 (see inset (a) in Figure 7). These bands are slightly shifted with respect to the GO single compound. However, we highlight the displacement of the G band from 1580 cm -1 to 1593 cm -1 ; this shifting is attributed to the regeneration of double bonds on the carbonaceous sheets that resonate at higher frequencies [55][56][57] suggesting that the GO sheets in our composites have been partially reduced.
The intensity ratio (ID/IG) of D band to G band of the GO is about 0.96 indicating a minimum concentration of defects in the carbonaceous sheets. Hydrothermal treatment at 180 ˚C for 6 h decreased the ID/IG ratio to 0.90 (Figure 4(b)). This suggests that the hydrothermal reaction, besides dehydrating/reducing the GO is also able to recover the aromatic structures by repairing defects. Therefore, we could conclude that the hydrothermal reduction route is more effective in repairing the sp2 network than the reduction process using other solvents [33]. An appropriate variation of the hydrothermal process temperature would allow us to control the extent of conversion of GO to graphene. The small S1 at 2701 cm -1 and S2 at 2900 cm -1 bands included in the 2D band (see inset (b) in Figure 7) suggest the presence of graphitic material of low crystallinity, the loss of functional groups and the consequent formation of C-H new bonds [58]. The TiO2 nanoparticles could improve GO dispersion and the defects concentration could diminish by their restructuration.

G. XPS spectra
The chemical state of the GO, TiO2 and the composites rGO/TiO2 in 1:1 and 1:2 in wt were analyzed by the XPS technique. These spectra were excited with X rays (1486.6 eV). Fig. 8a shows Oxygen XPS spectra, as a constituent element in: TiO2, GO and in the composites rGO/TiO2 in 1:1 and 1:2 in wt. Figure 8(b) shows the XPS spectra of Titanium as a constituting element in TiO2, rGO/TiO2 composites 1:1 and 1:2 in wt. Curve fitting was performed to all spectra using a Gaussian-Lorentzian peak shape after performing a Shirley background correction.
According to the XPS Wagner Handbook [59] the Ti 2p spectrum in the region 450-470 eV consists of the Ti 2p3/2 at 458.5 eV and Ti 2p1/2 at 468.2 eV binding energy. The energy difference between these two peaks is ΔE=5.66 eV. Figure 8(a) shows the XPS spectral peaks Ti 2p in TiO2, rGO/TiO2 composite 1:1 in wt and rGO/TiO2 composite 1:2 in wt. The XPS spectral peaks in TiO2 for the Ti 2p3/2 and Ti 2p1/2 are located at 458.72 and 464.38 eV respectively. The energy difference between these two peaks is ΔE=5.66 eV. The XPS spectral peaks in the rGO/TiO2 composite 1:1 in wt for Ti 2p3/2 and Ti 2p1/2 are located at 459.8 and 465.5 eV respectively, the energy difference between these two peaks is ΔE=5.7 eV and the XPS spectral peaks in the rGO/TiO2 composite 1:2 wt% for Ti 2p3/2 and Ti 2p1/2 are located at 459.00 and 464.72 eV respectively, the energy difference between these two peaks is ΔE=5.72 eV. According to these data we conclude that both composites contain TiO2 and the Ti 2p position remains constant when the concentration of TiO2 is varied to form the composites.
In order to observe the Ti-O bond evolution in the composites, in Figure 8(a) overlapped O 1s XPS spectra of the: TiO2, rGO/TiO2 in 1:1 in wt, rGO/    9 shows XPS spectra of C 1s as a constituting element of: a) GO, b) rGO/TiO2 composite 1:1 in wt and c) rGO/TiO2 composite 1:2 in wt. The C 1s in the XPS spectrum of GO shows 3 components: at 284.6 eV (FWHM=1.4 eV), 286.5 eV (FWHM=1.2 eV) and at 287.6 eV (FWHM= 2.0 eV). These peaks correspond to: nonoxygenated rings (284.6 eV), that include C-C, C=C, and C-H bonds; the C-O bond in the C-O-C or C-OH group (286.5 eV); and C=O bond (287.6 eV). The presence of these groups suggests a considerable degree of oxidation of the GO nanosheets [33].
High-resolution X-ray photoemission C 1s spectra of the composites show a significant decrease of oxygenated carbon related signals in the range 286-289 eV after hydrothermal process (Figure 9 (b,c) ) confirming that most of the epoxide, hydroxyl, and carboxyl functional groups were successfully removed. XPS spectra of the hydrothermally treated sample show that this method does not incorporate impurities, and the only elements present are oxygen and carbon. The hydrothermal conversion method to reduce GO with TiO2 has several advantages over the common chemical reduction processes, such as, 1) it requires very simple experimental setup, that is, basically an autoclave; 2) it has good upward scalability and it is industrially compatible with batch processing; 3) it is intrinsically pure because it utilizes only water instead of Hydrazine or Sulfonate to reduce GO, which unavoidably introduces noncarbon impurities into the treated GO [60]; 4) the closed system of relative high temperature and internal pressure promotes the recovery of π-conjugation after dehydration, which is favorable for minimizing defects [61]; and 5) engineering the parameters of temperature and pressure allows to control the degree of reduction of GO.

IV. CONCLUSION
We have synthesized GO following a safer and optimized modified Hummers' method [31]. We prepared the rGO/TiO2 1:1 in wt and rGO/TiO2 1:2 in wt using the hydrothermal method. HRTEM images of TiO2 powder show particles of about 20 nm forming agglomerations. Similar TiO2 clusters are observed in the composites but the size of the TiO2 particles is increased to about 40 nm, that is, the particles coalescence during the hydrothermal process. When the proportion of TiO2 is increased to form the rGO/TiO2 1:2 in wt composite the conglomerations get larger causing a nonhomogeneous distribution of the adsorbed TiO2 on the GO sheets. FTIR spectra of the rGO/TiO2 1:1 and 1:2 in wt suggest that during the solvothermal process the GO get reduced, that is, the GO loses oxygenated groups and consequently the sp2 Carbon bonds are increased. This effect is indicated by the intensity increment of the band at 1600 cm -1 corresponding to C=C bonds. Also we could infer that TiO2 binds the GO surface by means of C-O bonds as suggested by the appearance of the band at 1734 cm -1 of both spectra. According to the Raman spectra we conclude that the TiO2 in the rGO/TiO2 composites is in anatase phase. The Raman spectrum of GO shows the D and G bands typical of carbonaceous materials, and the corresponding bands appeared in the Raman spectra of the rGO/TiO2 composites except that they are slightly shifted to lower energies, suggesting that the GO sheets in our composites have been partially reduced.
According to the XPS spectra of TiO2 and the composites, we conclude that both composites contain TiO2 and that the Ti 2p energetic position remains constant when the concentration of TiO2 is doubled. The Ti-O bond is shifted toward greater energies (530.90 eV) when Ti-O-C bonds are generated, while the Ti-O bond in the composite bearing oxides shifts to the Ti-O binding energy in TiO2 when the amount of this compound is increased in the composite screening the Ti-O-C bond. The XPS analysis allowed us to find out the chemical state of the GO sheets. High-resolution X-ray photoemission C 1s spectra (XPS) of the composites show a significant decrease of oxygenated carbon related signals at 286-289 eV after hydrothermal process, confirming that most of the epoxide, hydroxyl, and carboxyl functional groups were successfully removed. XPS spectrum of the hydrothermally treated sample shows that it has no impurities, and the only elements present are oxygen and carbon.
The main conclusion of this work is that GO sheets of good quality have been obtained, TiO2 particles were anchored to the GO sheets by means of a hydrothermal process and the GO sheets have turned out partially reduced.